Engine emission control systems may include one or more exhaust catalysts to address the various exhaust components. These may include, for example, three-way catalysts, NOx storage catalysts, light-off catalysts, SCR catalysts, etc. Engine exhaust catalysts may require periodic regeneration to restore catalytic activity and reduce catalyst oxidation. For example, catalysts may be regenerated by injecting sufficient fuel to produce a rich environment and reduce the amount of oxygen stored at the catalyst. As such, fuel consumed during catalyst regeneration can degrade engine fuel economy. Accordingly, various catalyst regeneration strategies have been developed.
One example approach is shown by Goerigk et al. in U.S. Pat. No. 6,969,492. Therein, an emission control device includes catalytic converter stages generated by at least two catalysts arranged in series. Specifically, the catalytic stages include a three-way catalyst arranged in series with (e.g., upstream of) a NOx reduction catalyst. The different ammonia storage performance of the different catalysts enables NOx reduction to be improved and reduces the need for catalyst regeneration. Another example approach is shown by Eckhoff et al. in WO 2009/080152. Therein, an engine exhaust system includes multiple NOx storage catalysts with an intermediate SCR catalyst, and an exhaust air-to-fuel ratio is continually alternated between rich and lean phases based on differences between an air-to-fuel ratio upstream of a first NOx storage catalyst and an air-to-fuel ratio downstream of a second NOx storage catalyst.
However, the inventors herein have identified potential issues with such approaches. Catalyst regeneration strategies are not only dependent on the specific configuration and nature of the different exhaust catalysts in the emission control device, but for engine systems configured with variable displacement engines, the regeneration is also affected by the engine operating mode (that is, VDE or non-VDE mode). In particular, during a VDE mode of operation, when fuel is shut off to selected cylinders, the engine still spins a few more times. This spinning pumps air over an exhaust three-way catalyst, causing the catalyst to become oxidized and degrading its ability to reduce NOx when the cylinders are reactivated. While enrichment can be used to regenerate the three-way catalyst, the enrichment not only leads to a fuel penalty but also generates more ammonia which has to be dissipated from the three-way catalyst before the engine can re-enter the VDE mode again. As such, when there is no SCR catalyst in the engine system, NH3 is produced by the close-coupled three-way catalyst during rich engine operation and stored on the downstream underbody underbody three-way catalyst due to its cooler environment. However, when the engine is operated lean, the stored NH3 can be oxidized to NO by the underbody three-way catalyst. Herein, by placing an SCR catalyst upstream of the underbody three-way catalyst, it allows the SCR catalyst to catch the NH3 and prevents the underbody three-way catalyst from oxidizing it to NO during lean engine operation. As such, delays in entering or exiting the VDE/non-VDE modes can degrade engine performance. In particular, the delay in entering VDE mode to allow the stored NH3 to dissipate from the underbody three-way catalyst degrades the fuel economy as it limits how often we can enter VDE mode. As another example, VDE engines may include cylinder groups having dedicated exhaust catalysts as well as common exhaust catalysts. Based on whether a particular cylinder group is being deactivated or reactivated, a regeneration of the affected exhaust catalysts may need to be adjusted. This makes a regeneration strategy for the VDE engine more complex.
In one example, some of the above issues may be at least partly addressed by a method for an engine comprising, selectively deactivating one or more engine cylinders via deactivatable fuel injectors, and during cylinder reactivation, adjusting a combustion air-to-fuel ratio based on a change in ammonia content stored in a first exhaust catalyst, the change occurring during selective cylinder deactivation immediately preceding the cylinder reactivation. The adjusting may be performed until a regeneration state of one or more of a second exhaust catalyst and a third exhaust catalyst is higher than a threshold. In this way, ammonia generated during stoichiometric or rich engine operation can be stored on the first exhaust catalyst and advantageously used during a change from VDE to non-VDE mode to reduce the regeneration requirement of the second and third exhaust catalysts. Specifically, the stored ammonia can be used to reduce exhaust NOx while the third close-coupled catalyst is being reduced by the slightly rich exhaust to reduce the active regeneration requirement of the third close-coupled three-way catalyst. The second underbody three-way catalyst may be regenerated by the slightly rich engine exhaust. Active regeneration of the second underbody three-way catalyst may be performed if the vehicle driver requests high load engine operation.
In one example, a variable displacement engine may include distinct groups of cylinders coupled to a common exhaust manifold underbody. The underbody may include a first, SCR exhaust catalyst coupled upstream of, and in face-to-face brick contact with a second, three-way exhaust catalyst. As such, each of the first and second exhaust catalysts may be downstream of a third close-coupled three-way exhaust catalyst. During non-VDE operation, ammonia generated by the close-coupled exhaust catalyst can be stored in the first, SCR catalyst, and retained thereon during a subsequent shift to VDE mode of operation. An air-to-fuel ratio during the non-VDE mode may be adjusted to be stoichiometric, or richer than stoichiometry, to store a desired amount of ammonia at the first catalyst by the time a shift to VDE mode is performed. By storing the generated ammonia on the first, SCR catalyst, ammonia storage on the second three-way catalyst is reduced, thereby also lowering unwanted oxidation of ammonia to NOx at the second catalyst during the VDE mode of engine operation. During a subsequent return to non-VDE operation, the ammonia retained on the first, SCR catalyst may be used to reduce NOx species, while an air-to-fuel ratio is adjusted based on the ammonia content remaining on the first, SCR catalyst. The ammonia content may have changed during the VDE mode. In particular, the ammonia content may have changed based on a duration of operation in the VDE mode as well a degree of catalyst cooling incurred during the VDE mode of operation. In particular, the ammonia content of the SCR catalyst may have changed at least due to oxidation of the ammonia by the SCR catalyst in the lean environment (at temperatures above 300° C.) and due to SCR catalyst heating.
In this way, an air-to-fuel ratio may be adjusted while cylinders are not deactivated to charge an underbody exhaust SCR catalyst with ammonia and protect an underbody three-way catalyst from being charged with the ammonia. By using the stored ammonia during a subsequent cylinder reactivation from VDE mode, an amount of fuel required to regenerate the close-coupled three-way catalyst can be reduced, providing fuel economy benefits.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.